The present invention relates to a radio antenna arrangement which includes an arrangement of conductors disposed on a vehicle window to act as a radio signal receiving element, such an antenna arrangement being referred to generally herein as a xe2x80x9cscreen antennaxe2x80x9d. More particularly, the invention relates to such antennas for receiving signals in the LF and MF bands.
An antenna for receiving low and medium frequency radio transmissions (in the range 0.14 MHz to 1.6 MHz, which is conventionally abbreviated to LF and MF) for use in a vehicle has traditionally been provided by a whip antenna. Functionally, such an antenna acts as a voltage source with negligible impedance in series with a capacitor, this voltage being fed to a radio receiver along a coaxial feed cable which presents a capacitive coupling between the signal and the vehicle body.
A radio receiver which is intended for use in a vehicle is thus most typically configured for optimum performance when fed with signals from a whip antenna connected to it through up to 4 m of coaxial feeder, since this is the most commonly encountered arrangement. The receiver expects to xe2x80x9cseexe2x80x9d a pure capacitance on its antenna input.
A disadvantage of such an arrangement is that the capacitance of the feeder causes attenuation of the signal which may be frequency-dependent, depending on the radio input impedance. However, the signal output by a whip antenna is typically sufficiently large that the strength of signal presented to the receiver is sufficient, so the disadvantage is not apparent.
If the whip antenna is replaced by a screen antenna, the loss which occurs in the feeder may be of greater significance. The output from a screen antenna is typically less than that of a whip antenna, with the result that the signal presented to the receiver may be too low to for adequate performance of the receiver. Additional complications arise from the need to provide a connecting module between the screen antenna and the feeder in order to separate the received signals from the DC supply of heating power to the screen heater.
One solution to the problem of isolating the RF signals from the DC supply was to provide a unity-gain amplifier to supply signals from the antenna to the feeder. However, this gave rise to difficulties, in that the relatively low output impedance of the amplifier caused the input seen by the receiver to deviate substantially from the ideal pure capacitance. The result of this was that the receiver was operating with a source impedance different from that with which it was designed to operate, the consequence being that noise or instability could result.
It is well-known that impedance matching between components can be achieved through use of an intervening filter network. A proposed solution to the disadvantages discussed above connects the antenna to the feeder through a band-pass filter configured to provide beneficial impedance matching. However, the response of such filters is not constant over the frequency range of LF and MF radio signals, and they do not present the desired, purely-capacitive load to the receiver. An example of this type of arrangement is disclosed in EP-A-0410705, which also discloses the further refinement of including the capacitance of the feeder in the filter network, so as to reduce the disadvantageous effect of that capacitance.
It is an aim of the invention to provide an antenna arrangement whereby the effect of loss due to capacitance in the coaxial feed can be minimised, and good matching with, and signal transfer to, the receiver achieved, without the need to resort to use of an active circuit.
In arriving at the present invention, it was realised that an aim to be achieved was to transfer a voltage signal from a voltage source with a negligible resistive component (the screen antenna) to a load with a high input impedance (the receiver).
According to a first aspect of the invention there is provided a matching circuit having a signal input and a signal output, the matching circuit being operative to receive on its signal input an MF or LF signal output of a screen antenna and for feeding a signal though its signal output via a capacitive feeder to a radio receiver, the matching circuit comprising components which operate in conjunction with capacitances in components connected to the signal input and the signal output to form an internally-loaded band-pass filter network having a pass-band which covers a range of frequencies to be received by the receiver, and being a predominantly capacitive output over the pass band.
By this arrangement, the capacitances of the feeder and of the antenna itself are absorbed into the filter network, and rather than causing stray and undesirable effects, they contribute towards the desired and predictable functioning of the antenna system.
The network may have a Chebychev, Butterworth or elliptic function characteristic over the pass-band.
The network may be equivalent to a singly-terminated network optimised for transmission of signals between a signal source of defined resistance and a load of negligible or zero resistance. The network may be an even-order filter.
In embodiments according to the last-preceding paragraph, the network may be configured to give optimal input voltage/output current transfer into a short circuit.
In preferred embodiments, the network has a voltage output derived across a capacitance, the capacitance being the last element in the filter, and the capacitance being completely or partially constituted by the capacitance of a feeder to which the signal output of the matching circuit is intended for connection. In this manner, the capacitance of the feeder is a properly functioning part of the network.
In embodiments of a matching circuit of the present invention for use between an output amplifier of a screen antenna and a coaxial feeder connected to a radio receiver, the matching circuit may comprise an odd-order internally-singly-loaded filter network optimised for current transfer into a short circuit.
In embodiments according to the last-preceding paragraph, the last element in the network may be a series capacitor into a short circuit to which the network is connected, an output voltage of the network being derived across the capacitor, and that capacitor being at least in part constituted by the capacitance of a coaxial feeder.
From a second of its aspects, the invention provides MF and/or LF radio reception apparatus comprising an antenna having a signal output and being formed from a pattern of conductive elements applied to a glass pane, an amplifier having an input connected to the signal output of the antenna and having a signal output, a matching circuit according to any preceding claim having its signal input connected to the signal output of the amplifier, a radio receiver having a signal input, and a coaxial feeder connected between the signal output of the matching circuit and the signal input of the radio receiver.
Most typically, in arrangements according to the last-preceding paragraph, the radio receiver is installed in a vehicle and in which the glass pane is a window of the vehicle.
From a third of its aspects, the invention provides a method of optimising a matching circuit for use between an output of a screen antenna and a coaxial feeder connected to a radio receiver for reception of signals over a defined band in the LF and MR range the method comprising the steps of:
(a) devising a base circuit having optimal voltage transfer from a voltage source in series with an impedance to an open circuit over the defined band;
(b) deriving the dual of the base circuit being a bandpass filter circuit arranged such that optimal current flows in a short circuit, its signal output being derived across an output capacitor of value substantially equal to or greater than the capacitance of the feeder, the capacitor being the last element in the bandpass filter circuit;
(c) constructing a circuit omitting the output capacitor, the feeder being connected in substitution for the output capacitor whereby an output of the circuit is derived across the capacitance of the feeder. A matching circuit produced in accordance with the method is also provided.
By such a method, a matching circuit is obtained which is optimised for a particular application and which takes account of the effects upon its performance of surrounding components. The method defined above has the purther property of providing a filter by means of which the reactive component of an impedence experienced by a receiver connected to the feeder has a constant sign (i.e. is either purely capacitive or purely inductive) over the defined band.